Cell Membrane and its Functions

Cell Membrane

The cell is enveloped and thus separated from its surroundings by a thin wall contains a rigid framework of polysaccharide chains crosslinked with short peptide chains. Its outer surface is coated with lipopolysaccharide. Cell membrane is also called as plasma membrane (or) plasma lemma. The pili, not found in all bacteria have extensions of the cell wall. The cell membrane contains about 45% lipid and 55% protein. The cell membrane or plasma membrane have an average thickness of 75A°. The principal lipids are phospholipids, sphingolipids and cholesterol. An important feature of these lipids is they are composed of hydrophobic (water - insoluble) hydrocarbon sections and hydrophilic (water soluble) units. The latter include charged units (eg. phosphate or amino groups) and uncharged units (eg. hexose). In water, such compounds orient themselves in such a way that only the hydrophilic section is exposed to water. The hydrophobic components of individual molecules tend to contact with other; this is accomplished either by arrangement into micelles or by the formation of bilayers.

Two types of poteins are found in cell membrane; viz. intrinsic or integral and extrinsic or peripheral (Fig. 1.3).Integral proteins are either partially or totally immersed in the lipid bilayer and difficult to remove by any means other than the distruption of the membrane with a detergent. Peripheral proteins are bound only to the surface of membrane and interact only with the hydrophilic groups and therfore are readily removed by extraction with an aqueous medium. The model of arrangement of lipids and proteins in the memberane is known as fluid mosaic model.

Functions of cell membrane

·           The cell membrane performs a number of important functions It holds the cell together

·           The membrane is a selectively permeable boundary which allows water, certain required nutrients and metal ions to pass freely It secrete waste products

·            It keeps out toxic materials

·           It contain receptors to bind certain regulatory substances such as hormones which regulate the various metabolic activities.